new preloader
remove the old preloader
[TODO] Bug 1077652 - Update browser_newtab_background_captures.js to work with the new preloader
[TODO] Bug 1077652 - Revert change from bug 794041 that makes test_docload.html expect accessibles in the hidden window
[TODO] Bug 1077652 - Let the test_tabbrowser.xul a11y test know about the extra preloaded document